This application claims priority to an application entitled xe2x80x9cApparatus and Method for Measuring Residual Stress and Photoelastic Effect of Optical Fiberxe2x80x9d filed in the Korean Industrial Property Office on Jan. 16, 2001 and assigned Ser. No. 2001-2366, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to an apparatus for measuring optical fiber. More particularly, the present invention relates to an apparatus and method for measuring residual stress and the photoelastic effect of an optical fiber.
2. Description of the Related Art
In general, a process for manufacturing an optical fiber includes a process for manufacturing an optical fiber preform and a process for drawing a manufactured optical fiber preform in a strand of optical fiber. Stress occurs in the process for manufacturing optical fiber. The stress may remain in the optical fiber without being removed even after manufacturing the optical fiber. In this case, the stress is called residual stress. It is important to measure the residual stress in view of the manufacture of an optical fiber having high quality, an optical fiber grating, and development and study of a special optical fiber.
The residual stress occurring in a process for drawing an optical fiber at a high temperature increases the loss due to scattering of the optical fiber and causes variation in a refractive index due to a photoelastic effect. Accordingly, to manufacture an optical fiber having high quality, it is necessary to develop the optical fiber manufacturing(drawing) technique that reduces the residual stress. To this end, it is essentially required to develop an equipment that can measure residual stress of an optical fiber.
Furthermore, the periodic removal of some of the residual stress of an optical fiber using a CO2 laser has been recently developed for a long period fiber. Also, there is provided a long period Bragg grating based on UV irradiation. As a means of analyzing the principle of the long period Bragg grating, an apparatus for measuring residual stress distribution is used to prove compaction effect of glass due to UV irradiation.
To study and improve spectral transmission characteristic of the Bragg grating, it is necessary to measure variation effect in a refractive index due to variation of residual stress and the distribution in which the residual stress is periodically removed in a longitudinal direction of the optical fiber.
In addition, it is necessary to deeply study variation in a refractive index due to residual stress, i.e., variation effect of a doping material due to photoelastic effect. To this end, it is necessary to develop an equipment that can three-dimensionally measure and observe residual stress of optical fiber and photoelastic effect.
Residual stress of an optical fiber or optical fiber preform is measured using a photoelastic effect. The photoelastic effect means variation in a refractive index of a medium depending on a direction of stress that remains in the medium. The refractive index of the optical fiber or the optical fiber preform has a variable value depending on a polarized direction of light. When light of two orthogonal polarized components passes through a side vertical to an axis direction of the optical fiber preform or the optical fiber, phase difference of the light occurs depending on the polarized direction of light due to the photoelastic effect by the residual stress. By measuring the phase difference using a polariscope, the range of the residual stress can be identified. Examples of measuring the residual stress of the optical fiber preform and the optical fiber based on the aforementioned conventional method will be supposed in three representative papers as follows.
1. P. L. Chu and T. Whitebread, xe2x80x9cMeasurement of stresses in optical fiber and preformxe2x80x9d, Appl. Opt., 1982, 21, pp 4241xcx9c4245.
In this paper, a method and theory for optically measuring residual stress of optical fiber using photoelastic effect has been first suggested in detail and residual stress profile of optical fiber and its preform has been measured.
2. Th. Rose, D. Spriegel and J. R. Kropp, xe2x80x9cFast photoelastic stress determination application to monomode fibers and splices,xe2x80x9d Meas. Sci. Technol. 4, 431xcx9c434, (1993).
In this second paper, based on residual stress of a monomode fiber and photoelastic effect used to irradiate stress occurring during fusion splice, an optical measuring apparatus has been developed.
3. K. W. Raine, xe2x80x9cA microscope for measuring axial stress profiles in optical fibres with high spatial resolution and low noise,xe2x80x9d 4th optical fibre measurement conference (NPL Teddington UK), 269, (1997).
In this third paper, there has been suggested a method and apparatus for measuring residual stress of optical fiber having high resolution by applying CCD and half-shade method to a measuring method based on the existing photoelastic effect.
To measure residual stress of an optical fiber, residual stress of an optical fiber preform (diameter of 4 cm) having a greater sectional area than the optical fiber is measured so that the residual stress of the optical fiber is analogized. However, since the optical fiber preform has only thermal residual stress, a problem arises in that dynamic residual stress cannot be measured. Accordingly, to measure the dynamic residual stress, the optical fiber should directly be measured under the conditions of high resolution and super-accuracy considering its size (cladding diameter is 120 xcexcm and core diameter is 8 xcexcm).
However, the existing measuring method has various technical problems such as relatively smaller phase difference than the optical fiber preform, difficulty in magnifying an image, imaging error due to relative light intensity of image background.
When residual stress of the optical fiber preform is measured, since an optical path of light that passes through the optical fiber preform is long, the phase difference between two orthogonal polarized lights is estimated at 180xc2x0 or less. However, when residual stress of the optical fiber is measured, the optical path has a length of {fraction (1/100)} as compared with the optical fiber preform. In the optical fiber, the phase difference of lights is estimated at 2xc2x0 or less. Accordingly, to measure residual stress of the optical fiber, a measuring apparatus having polarization resolution of a minimum 0.1xc2x0 is required.
Since a diameter of the optical fiber is 125 xcexcm, it is necessary to magnify the diameter to form an image. In the existing method of measuring residual stress of the optical fiber preform, it is difficult to closely adhere an objective lens to the optical fiber due to a rotation device of a polarization analyzer and a quarter wave plate on which light that passed through the optical fiber is entered. It is also difficult to raise magnification in technical aspects. Consequently, there results in that a magnified image of the optical fiber is distorted by polarizers. Accordingly, it is necessary to develop a measuring apparatus that minimizes polarized errors and wave plate distortion of light that passed through the optical fiber.
It is, therefore, an object of the present invention to provide an apparatus and method for measuring residual stress and photoelastic effect of optical fiber, which can minimize wave plate distortion and polarization error of light that passed through the optical fiber.
It is another object of the present invention to provide an apparatus and method for measuring residual stress and photoelastic effect of optical fiber, in which provides a simple structure at low cost.
To achieve the above objects, there is provided an apparatus for measuring residual stress and photoelastic effect of optical fiber including a rotating light diffuser for providing temporally averaged uniform image by restraining spatial coherence of light emitted from a light source to be completely scattered, a collecting lens for aligning the light passed through the rotating light diffuser in an optical fiber, a linear birefringence device for linearly controlling the polarization state of the aligned light input from the collecting lens, a condenser for condensing the light passed through the linear birefringence device on a place where the optical fiber is located, a fixed polarizer for transforming the polarization state of the light passed through the optical fiber to the intensity distribution by allowing the light emitted from the linear birefringence device to transmit the optical fiber, an objective lens for obtaining a magnified optical fiber image having the intensity distribution of the light transmitted the optical fiber, and a unit for measuring transmittance variation given to the optical fiber depending on locations.